The length of time you will be in Barcelona will dictate what you have time to see and how long you will be able to spend at each location.
I wrote a post in 2016 on TOP PLACES TO SEE IF YOU ONLY HAVE 1 DAY IN BARCELONA
If you only have a short period of time to visit (for example, you arrived by cruise ship), you should purchase tickets for the hop-on-hop-off bus and see as much as possible.